Printing – Stenciling – Processes
Reexamination Certificate
1999-04-16
2001-01-09
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Processes
C101S127000, C101S128210
Reexamination Certificate
active
06170394
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a plastic mask for paste printing for forming patterns by use of an ink, an adhesive agent, a solder paste, or a paste-like resin (hereinafter referred to as the paste) on a printing material, more particularly to a method of fabricating a plastic mask for such paste printing, which is carried out by forming a pattern of through-holes in a plastic sheet by excimer laser abrasion.
The present invention also relates to a plastic mask fabricated by the above-mentioned method, and also to a paste printing method by use of the plastic mask.
2. Discussion of Background
For forming a printed pattern on a printing material with the above-mentioned paste by use of a printing mask, the following method is generally used:
A mask with a pattern of through-holes is brought into close contact with a printing material, and a paste is spread on the surface of the printing mask by a squeegee so as to fill the through-holes with the paste. An excessive paste is then removed from the surface of the printing mask.
The printing mask is then separated from the printing material in such a manner that the paste passes through the through-holes and remains in the same pattern as that of the through-holes formed in the printing mask on the printing material, whereby a paste pattern corresponding to the pattern of the through-holes is printed on the surface of the printing material.
Such printing masks for use in practice can be roughly classified into screen masks and metal masks.
As screen masks, there are known, for example, punch press mask, YAG laser mask, etching mask, additive mask and half-etching mask.
The punch press mask was developed as a solder paste printing mask for fabricating electronic parts. The punch press mask can be prepared by forming round through-holes in a metal sheet with a punch.
This printing mask can be easily and quickly prepared by utilizing NC data of a mounter used in a mounting process with high through-hole making speed.
This printing mask has the advantages over other printing masks that the production method is simple and therefore the printing mask can be produced at extremely low cost. Furthermore, the apparatus for producing the printing mask is also inexpensive and small-sized and therefore can be brought into and used in a printing facility without difficulty.
Despite such advantages, this printing mask has the shortcomings that upper edge portions of the through-holes tend to be rounded, and rough broken surfaces or burrs tend to be formed in the lower edge portions of the through-holes, so that the paste filled in the through-holes cannot smoothly pass through the through-holes and therefore the printing quality is poor.
A minimum diameter of the through-hole that can be formed is about 0.3 mm. For example, in the solder paste printing for fabricating electronic parts, a minimum pitch will be about 0.5 mm in QFP (quad flat package) at best.
A YAG laser mask is prepared by fusing a metal sheet (usually SUS) with YAG laser and fused portions in the metal sheet are blown off by use of an assist gas to form through-holes in the metal sheet.
In the case of the YAG laser mask, through-holes are made in accordance with the NC data or the like, so that the YAG laser mask has the advantage that the mask can be produced in a short time with a minimum dispersion in quality, but has the disadvantages that the inner walls of the through-holes are rough, and dross is deposited at the edge portions of the through-holes, so that the printing quality obtained by the YAG laser mask is slightly inferior to that obtained by other printing masks.
Furthermore, convex portions are formed in the edge portions by the deposition of the dross thereon, and the thus formed convex portions tend to make imperfect the contact of the printing mask with a printing material during the printing process. If such imperfect contact takes place, it is necessary to remove the convex portions from the edge portions by grinding or polishing the surface of the printing mask.
An etching mask is prepared as follows: A photosensitive resin is coated on both sides of a metal plate to form resist layers thereon. The resist layers are exposed to light through a photomask on which through-holes are depicted in a pattern (hereinafter referred to as the through-hole pattern depicted photomask) and are then developed by removing the portions corresponding to the through-hole pattern from the resist layers. This metal plate is then subjected to etching, whereby through-holes are made in the same pattern as the above-mentioned through-hole pattern in the metal plate. Thus, an etching mask for printing is prepared. In the thus prepared etching mask, etching is carried out on both sides of the metal plate for making the through-holes therein, so that a convex portion is formed at the central portion of the inner wall of each through-hole. Furthermore, the inner wall of the through-hole is rough, so that a paste cannot smoothly pass through each through-hole. The result is that the printing quality obtained by the thus prepared etching mask is poor.
An additive mask is prepared as follows: A relatively thick photosensitive resin layer or dry film layer is provided on a plating substrate. The photosensitive resin layer is exposed to light through a through-hole pattern depicted photomask and developed by removing the portions in the photosensitive resin layer except the portions corresponding to the through-hole pattern. Thus, the photosensitive resin layer portions remain in the through-hole pattern on the plate substrate. A metal plate, for example, made of nickel, with an appropriate thickness, is formed by plating around the photosensitive resin layer portions which remain in the through-hole pattern. The metal plate is then removed from the plating substrate, and the photosensitive resin layer portions are dissolved away from the metal plate, whereby a metal plate with through-holes which correspond to the removed photosensitive resin layer portions is prepared as a printing mask.
The inner walls of the through-holes formed in the above metal plate are smoother than any of the inner walls of through-holes formed in metal masks prepared by any other methods and accordingly, the printing quality obtained by the metal plate is relatively good. However, occasionally, the inner walls of the through-holes are roughened by the over-exposure of the dry film, which may be referred to as “attack phenomenon”.
It is considered that the “attach phenomenon” is caused as follows. It is necessary that the photosensitive resin layer or dry film for the formation of the through-holes in the printing mask be sufficiently thicker than the additive mask. Occasionally, the thickness of the photosensitive resin layer may be 200 &mgr;m or more. Since this thickness is much greater than that of a photosensitive resin layer for the masking of an etching liquid, which is in the range of 20 to 30 &mgr;m, a sufficient amount of light must be applied so as to have the light reach the bottom portion of the photosensitive resin layer on the side of the substrate. For this reason, light with high intensity is applied to the uppermost surface of the photosensitive resin layer on the side of the photomask.
When the above-mentioned light applied to the uppermost side of the photosensitive resin layer causes halation within the photosensitive resin layer, the portions which should be shaded by the photomask are exposed to the light scattered from the adjacent portions within the photosensitive resin layer. As a result, the inner walls of the through-holes are roughened as mentioned above.
Furthermore, stepped portions are formed in the inner walls of the through-holes with the attachment of the dry film thereto while in use, whereby the printing quality is lowered.
The plated surface of the printing metal plate which comes into contact with a printing material tends to be roughened. In particular, when finely-divided particles are containe
Funk Stephen R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh Microelectronics Co. Ltd.
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